Maybe you should use another API/URL. This function below uses a different url that returns JSON data and seems to work better:

    public static string Translate(string input, string fromLanguage, string toLanguage)
    {
        using (WebClient webClient = new WebClient())
        {
            string url = string.Format("http://translate.google.com/translate_a/t?client=j&text={0}&sl={1}&tl={2}", Uri.EscapeUriString(input), fromLanguage, toLanguage);
            string result = webClient.DownloadString(url);

            // I used JavaScriptSerializer but another JSON parser would work
            JavaScriptSerializer serializer = new JavaScriptSerializer();
            Dictionary<string, object> dic = (Dictionary<string, object>)serializer.DeserializeObject(result);
            Dictionary<string, object> sentences = (Dictionary<string, object>)((object[])dic["sentences"])[0];
            return (string)sentences["trans"];
        }
    }

If I run this in a Console App:

    Console.WriteLine(Translate("How are you?", "en", "es"));

It will display

¿Cómo estás?